low airflow on York twinned furnaces

[chill67]

Md# GY9S120D20UP11J, furnaces are twinned together (5 ton evap. on each furnace) & sit on return box. Total cfm (checked with flow hood) with merv 7 filters, fan on high speed is 2458. Manual states 3400 cfm when twinned. Removed filters, cfm 3244.
Removed blower doors, cfm 3310
Heres my question: return box has two 16x25x1 filters (one under each furnace). The filter rack is built into the return box the furnaces sit on. The install manual states this md. furnace requires a 22x25x1 filter, I obviously have a lot of area blocked off due to smaller filter rack under each furnace. Is this most likely my problem with low airflow?
Sorry, no pics

Thanks for any input!

[HvAckid82]

If the system is designed around a .1 friction you could achieve 3200 cfm at 16x25x1 if the restriction is not too great. Your filter sounds like it is putting it over the edge.

[genduct]

You are stuck with the fan so make the duct more efficient

The filters are a source pressure drop as per your test so you can look at modifying the return plenum so you'll "see" more than the 2 filters.
OR you'll take a closer look at making the return duct that leads to the filters shorter/ more efficient, thereby allowing more of the static pressure be availiable for the supply side duct.

[chill67]

Sounds like I need to modify filter racks to 22x25 filters, I can gain approx. 300 sq. in. of return air if I do this. Have to figure a way to remove existing 16x25 racks, the way the return box was built it is part of the supporting structure to hold the weight of furnaces\evap. coils.

[klyons20]

Pull fan motors and clean secondary heat coil just above the fan! Use a coil cleaner.
This could be your problem

[chill67]

Equipment 6 months old, but good thought

[ctcont]

Make the return bigger use what the factory wants i had the same problem on a account i took over that was my fix and i also used the factory tech support